Slice 2 + 1b:
- Bill editor: VAT-rate field → VatRateSelect dropdown (mirrors the quote
editor); snapshots vatCode on create + carries it from a source quote.
- getQuoteById + the invoice serializer now return vat_code, so re-editing a
saved document preserves the snapshot instead of falling back to the
rate→code map. Payload types (quotes + bills) carry vatCode.
72 tests pass; build green.